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Hi there Im new here and made a cardinal sin of spelling Merckx wrong in my username

Im a bloke with height 173cm and I ride a Cannondale bike (size 54 ) which has a toptube length of 545mm. On the bike I ride with a deda 90mm stem set to the +6 degree angle as I like to ride long distances in comfort. My stem is a controltech type which has setback of 10mm. The 20mm setback length just had me positioned a little bit too far back and pushed my kops position back behind the center of the pedal spindle , the 10mm stem places my knee in exactly the right spot and there are no reach issues to the bars.

My question concerning the various tube lengths relates to a size 52 frame and its headtube length. I had been considering purchasing a 2nd bike for the winter season and saw some nice size 52 models and wondered how a size smaller frame would handle. The top tube on average measured around the 535mm ballpark which would neccessitate me using a 100mm stem which isnt a problem, but the headtube lengths tended to be in the 135mm range. Now that does cause a problem, my cannondale has a 155mm headtube and I use 25mm of spacers. This places my 105 shifters at a very comfortable height for me and Im thinking that on a 135mm headtube I would need a skyscraper of spacers and I do not like that idea.

Biologically I have short arms, 30 inch inseam and a long upper torso and these body proportions make fitting a real headache. I like the look of the size 52 canyon roadlite, but that 135mm headtube length scares me off as low slung aero position riding is a style I find really uncomfortable. Using size 54 with bigger headtube gives me a more upright position and I could ride all day using it. In my position is a size 52 frame with 135mm headtube always going to present difficulties for me ?

While I understand that a majority of your question is around head tube height (which I too think you sorta answered yourself), that top tube length between 2 different bikes means little if you don't incorporate the seat tube angle into the equation.

Traditional wisdom (although it's challenged more these days) is that you should be more concerned about your top tube length, which affects your handling and weight distribution. WRT your head tube length, a few extra spacers won't affect too much.

The suggestion to flip the stem up won't work, since you've already posted a flippped-up +6 degrees. I suppose you could use a +10 degree stem, but that won't get you much more bar height.

You need to think about reach, not just TT length. In the case of the CAAD10, the TT is 1cm shorter, but the STA is 1 degree steeper, so the reach is only about 6mm shorter on the smaller frame. You would need about 10mm more seatpost setback.

C'adale publishes the stack and reach, but when comparing the reach, there's a correction needed for the differing stack heights. Once that's done, the 52cm frame would have a reach that's about 6mm shorter. You get the samer answer, using either method of figuring the reach.

I would suggest trying lower bars by flipping the stem down on your current bike. I've done that, going from a 9cm saddle to bar drop to 11cm and it didn't bother me. You never know unless you try it.

MerckzFan wrote:The 20mm setback length just had me positioned a little bit too far back and pushed my kops position back behind the center of the pedal spindle , the 10mm stem places my knee in exactly the right spot and there are no reach issues to the bars. .

That tells me a lot. Like you are new to cycling and maybe have placed too much store in the maths of bike fit. Your current set-up probably 'places your knee in exactly the right spot' if you were riding a time trial in the Tour de France. Comfort is obviously of importance to you and you should not overlook the importance of placing your saddle further behind the BB in an attempt to achieve greater comfort. Let trial and error be your friend.

Back to the point of your post: It is a simple analysis that with modern race frames these days, if you get one with the 'correct' top tube length, the short head tubes that these frames usually come with mean the fitting of a sizeable stack of spacers. Either that, or you go with the next size up and run a short stem. These general rules apply if you are a mere mortal and not some kind of fit and supple young rider. My point being: if we are talking 2cm size incraments, riders will often find that two sizes of frames will generally fit a rider these days ... but in different ways.

Either that, or seek out a more sportive frame aimed at the recreational cyclist.

I would tend to agree with Valbrona's conclusion. Go with what is comfortable. There is always the custom option, if you want.

As a comment, I disagree with the idea that modern frames have short head tubes in relation to the top tubes, rather, I think it is the opposite. Some frames offer 'pro' geometries, with shorter head tubes, but generally you will need to 'size down' and run a 130 or 140mm stem in order to get the bars low enough on a modern, monocoque frame.